Sammy Kitwara's story does not fit the usual narrative for top Kenyan marathoners, the one that begins with their first steps in the sport coming from miles run to and from school daily.

Kitwara's childhood home was in Tot, a village in the Rift Valley lowlands with a climate he said generally was too hot for running. He noted the area also was inhospitable to runners for another reason: violent clashes that flared frequently for more than a decade until early this century between members of his tribe, the Marakwet, and members of the nearby Pokot tribe.

So Kitwara, 28, the favorite in Sunday's Bank of America Chicago Marathon, did not begin running until after high school, when he moved 70 miles south to Eldoret, one of the Kenyan cities renowned for its runners.

"I saw people from Eldoret running on television and I became interested," Kitwara said. "So I went to Eldoret to try and started training slowly, slowly."

This was 2006. Within a year, Kitwara was doing so well in local cross-country races he came to the attention of Dutchman Frans Denissen, who became his agent in 2008.

"A great talent, with a late start," Denissen said.

Kitwara would finish fifth in the 10,000 meters on the track at the 2008 Kenyan Olympic trials and win that race at the 2009 Kenyan Championships, ostensibly becoming a top candidate for a spot on his country's team at the world meet. His decision to run — and win — the 10,000-meter Peachtree Road Race in Atlanta a week later led to a dispute with the Kenyan track federation, knocking Kitwara out of world team contention and convincing him to leave the track for road racing.

He worked up methodically over the next three years to the marathon distance, failing to finish his first attempt (Rotterdam, April 2012) but getting fourth at Chicago six months later. That was followed by a third in the 2013 Chicago Marathon and a second in 2014 in 2 hours, 4 minutes, 28 seconds, fastest of his seven career marathons and fourth fastest in the world last year.

Kitwara spent five years training under the direct and indirect guidance of Moses Kiptanui, three-time world champion in the steeplechase. Since 2013, he has devised his own training program but trains with a group of runners that sometimes includes Wilson Chebet, a podium finisher at the last two Boston Marathons.

"If I hadn't started running? I would be a farmer, married with a lot of kids," Kitwara said.

Kitwara lives in Eldoret with his wife, Sylvia, and their two children, 5-year-old Christian and 2-year-old Gynell.

He comes to Chicago after a sixth at April's London Marathon, where a hamstring problem knocked him out of contention at 18 miles. A convincing victory in last month's Luanda, Angola, half marathon told Kitwara the hamstring was fine.

•The lingering effects of a stomach virus will keep Joan Benoit Samuelson, 58, from pursuing her "30-for-30": a finish within 30 minutes of her winning time (2:21:21) in Chicago 30 years ago, which stood as the U.S. record for 18 years. Samuelson will decide Sunday whether she feels well enough to celebrate the anniversary by running with a less competitive mindset.